FSK is a frequency modulation method in which digital information is transmitted through discrete frequency changes of a carrier wave. The simplest FSK method is binary FSK (BFSK). BFSK uses a pair of discrete frequencies to transmit binary (0s and 1s) information. With this method, the “1” is generally called the mark frequency and the “0” is generally called the space frequency.
There are a variety of known techniques known for FSK decoding an FSK encoded signal. Most commonly used FSK/tone decoding techniques uses phase lock loops (PLLs) or dedicated tone decoders in the analog domain, window comparison in VHSIC Hardware Description Language (VHDL) in the digital domain, or digital logic based simple decoders for low frequency applications.
FSK decoding is generally relatively easy when the respective frequencies are widely spaced apart (e.g., (Highway Addressable Remote Transducer Protocol (HART) 1,200 Hz (8.3 ms)-2,200 Hz (4.5 ms) and the baud rate is low. But when the respective frequencies are closely placed and when the baud rate is high, FSK decoder design becomes complex especially for communication applications where noise margins are higher on received signals.
There are dedicated tone decoder devices generally available which can generally decode FSK modulated signals with above mentioned constraints. However, these dedicated tone decoder devices add to the cost and complexity of the design, including tuning to specific frequencies, sensitive to aging of components, temperature dependent variations. These dedicated tone decoder devices are generic in nature and specific requirements on frequency variance and the lock-in time for tones typically cannot be altered/customized beyond a certain level.